Worth it to put headers on a 318?

Worth it? As far as performance, probably not. You're committed to 600 CFM on the intake end. That means that you need to blow 300 CFM out of each manifold. I really don't know the flow rate of a 318 manifold but you'd think it has to be above 300. If not a set of 360 manifolds should certainly be above 300 CFM. I'd do that long before a set of headers.